1. Field of the Invention
This invention pertains generally to measuring and testing, and more particularly to measuring liquid levels using a float and sensor. The measuring apparatus is preferably used to detect fuel levels within a receptacle such as a fuel tank.
2. Description of the Related Art
Fuel level sensors typically found in automobiles provide many challenges to a designer trying to achieve enduring performance. Among the challenges are factors specific to the automotive environment, such as high vibration, frequent cycling including simple sloshing of fuel within a tank, and widely varying operating temperatures. Other challenging factors are specific to the automotive fuel system, primarily derived from the sender being exposed to a wide variety of fuels and additives. For example, both gasoline and alcohol are generally effective at dissolving grease compounds and lubricants, thereby preventing a designer from incorporating lubricants into the sender. Both gasoline and alcohol will also cause swelling in many plastic materials. Detergents are also generally a part of automotive fuels, once again preventing any incorporation of grease or lubricant. Off-the-shelf gasoline additives often include very powerful cleaners and solvating agents for purposes such as fuel varnish removal. These cleaners and solvating agents will also attack various plastics and remove grease and lubricants. Finally, challenges exist for the designer which are common to resistive type sensors in general, such as variations in contact resistance, wear, and corrosion. Each of these challenges tend to reduce the effective life of the sender and limit options available to a designer.
In recognition of the unusually harsh automotive fuel environment, the use of lubricants which is traditional in other resistive or contacting sensor environments is impractical. Furthermore, polymer-bound resistive compositions are susceptible to fuel components and fuel additives, and have therefore also proved to be unacceptable. As a result, cermet materials which are formed from glasses, ceramics, conductive metals and screening agents are the materials of choice for fuel senders. Cermet resistors are typically formed from either ruthenium dioxide or silver-palladium compositions that are mixed or compounded with specialty glasses. Often, the cermet conductors are formed from silver or silver-palladium conductors, also mixed or compounded with specialty glasses. The specialty glasses will frequently include significant amounts of aluminum oxide and silica, both which are known to form hard and potentially very abrasive glass compositions. The compositions are typically screen printed onto a refractory substrate such as aluminum oxide, and then fired at very elevated temperatures, often in the range of six hundred to one thousand degrees Centigrade. At the elevated temperatures, the specialty glasses will begin to reflow and sinter, as will the precious metal particles. The screening agents pyrolitically decompose, leaving little or no residue. Eventually a conductive network is formed within the cermet material, and, most preferably, the glass forms an adhesive bond with both the substrate and the metal particles. Important to note, however, is the fact that neither the glass or the metal will actually be fully molten. Rather, the sintering process involves a surface energy phenomenon wherein smaller particles tend to unite to form larger, usually very non-spherical masses. These cermet materials offer much advantage in stability and chemical resistance, being very nearly inert with regard to the fuel components. Furthermore, the cermet materials are very hard, thereby reducing wear of the resistor material during the large number of cycles required in an automotive fuel sender. To address contact resistance issues, the resistor element is frequently patterned with or segmented by conductor stripes or dots. The contactor, instead of sliding over and contacting relatively high resistance materials, is able to contact low resistance materials which thereby tend to lower contact resistance and contact resistance variations. Furthermore, by using the low resistance material for a contact surface, wear does not change the sensed output measurably.
Unfortunately, in spite of the many advantages inherent in cermet compositions and conductor stripes, automotive fuel senders continue to be plagued by early failures. Conductive traces are worn from the alumina substrates, resulting in poor or completely failed electrical contact and elevated resistance readings. Events of poor or lost electrical connection may be sensed by the engine computer signalling a service engine light, or may trigger inaccurate or erratic fuel level measurements.
As a result of the continuing difficulties, a number of efforts have been made to improve fuel level sensors and thereby extend operational life. One such example is illustrated in U.S. Pat. No. 5,169,465 to Riley, incorporated herein by reference. Therein, a process is disclosed for forming a relatively smooth layer of glass between stripes of conductive material. Riley achieves this by sinking a cermet conductive material into a glass dielectric layer during sintering. While this process was initially believed to offer a solution to extended life, the full benefit originally conceived was, in practice, never actually achieved.
Another example of efforts at improving sender life is illustrated in U.S. Pat. No. 5,341,679 to Walkowski et al, also incorporated herein by reference. In the Walkowski reference, auxiliary components known to affect the vital contactor-resistor interface are improved, to gain tighter control over and ultimately achieve ideal parameters. Once again, however, these attempts may have proved beneficial but senders continue to demonstrate reliability problems.
These prior art senders are illustrated in FIGS. 1 and 2 by exaggerated cross-section, wherein a sender 100 includes a resistor substrate 110 having patterned thereon thick film cermet conductors 120, 122, 124 which include microscopically rough surfaces 130, 132, 134. Surfaces 130, 132 and 134 may be formed form the same cermet material as base conductors 120, 122, 124, or, as is known in the prior art, these surfaces 130, 132, 134 may be a different composition. Sliding over surfaces 130, 132, 134 and dropping partially towards substrate 110 at gaps 112 and 114 is contactor 140. While contactor 140 is illustrated as a rake in FIG. 1, it will be apparent to those of ordinary skill that there are a wide variety of contactor geometries available in the art and the rake geometry illustrated in FIG. 1 is purely for exemplary purposes. As aforementioned in the Riley patent, the phenomenon which was believed to damage the variable resistor was the undulating, up and down motion shown by line 150 which represents the vertical travel of contactor 140 as contactor 140 is passed horizontally over a length of substrate 110. In the Riley patent the undulation of line 150 was reduced by forming a continuous glass layer and sinking conductives 120, 122 124 into the glass layer. Unfortunately, and as aforementioned, this did not provide the desired solution. By further magnifying a single trace in prior art FIG. 2, the surface valleys such as valley 131 and surface peaks such as 133 are more apparent. The difference between the peaks and valleys may be represented by the dimension R.sub.a, representative of surface finish, illustrated in FIG. 2. It has been determined by the present inventors, as will more fully be described hereinbelow, that the failure mechanism is not the undulation of contactor 140 illustrated by line 150, but instead is a result of the microscopic surface roughness represented by R.sub.a.